Download A force is a push or pull. An object at rest needs a force to get it

Force
A force is a push or pull. An
object at rest needs a force
to get it moving; a moving
object needs a force to
change its velocity.
The magnitude of a force can be measured using a spring
scale.
© 2014 Pearson Education, Inc.
Units of Force
●
●
SI unit of force is a Newton (N)
kg m
1 N ≡1 2
s
US Customary unit of force is a pound (lb)
●
●
1 N = 0.225 lb
See table 4.1
2
4.1 The Concepts of Force and Mass
A force is a push or a pull.
Contact forces arise from physical
contact .
Action-at-a-distance forces do not
require contact and include gravity and
electrical forces.
Contact and Field Forces
4
4.1 The Concepts of Force and Mass
Arrows are used to represent forces. The length of the arrow
is proportional to the magnitude of the force.
15 N
5N
 Newton’s First Law of Motion states:
An object will remain at rest or in motion
with constant velocity unless acted on
by a net external force.
 An external force is a force applied to
the object from some other object.
 force from an impact, gravity, air
resistance, etc
 Net force = 0 means acceleration = 0
Velocity = constant
same magnitude and same direction
4.2 Newton’s First Law of Motion
Inertia is the natural tendency of an
object to remain at rest in motion at
a constant speed along a straight line.
The mass of an object is a quantitative
measure of inertia.
SI Unit of Mass: kilogram (kg)
misconception: " Motion requires force " WRONG.
A puck initially hit will keep moving on the ice until it is
stopped by a wall or by a player.
Motion does not require force.
A net Force is only required to change the motion.
to speed up an object, to slow it down, to stop it , to get it to
move or to change its direction,
Mass is amount of “ stuff” or matter in an object.
Mass is a measure of inertia. How much an object
resists a change in motion. Its laziness to change
Its direction or speed.
Source: Conceptual Physics, Paul Hewitt
WHAT HAPPEN TO THE PASSENGERS
OF A CAR WHEN IT ABRUPTLY BREAKS ?
OR SPEED UP ?
WHY DO YOU NEED TO WEAR A SEAT BELT ?
HOW CAN YOU EXPLAIN THE TABLE TRICK
USING INERTIA PRINCIPLE ?
http://www.youtube.com/watch?v=vfnt8Sdj7cs
USING A THOUGHT EXPERIMENT, GALILEO REASONED
THAT ARISTOTLE WAS WRONG.
IF THERE IS NO NET FORCE, AN OBJECT IN MOTION
STAYS IN MOTION, AT THE SAME SPEED AND
THE SAME DIRECTION. Describe this experiment:
An elephant has a large inertia and can run
very fast. What could you do to escape a
charging elephant ? (using Newton;'s 1st law)
stinger missiles are heat seeking missiles.
They detect infra red and can hit planes.
They reach them because the " feel " the heat.
If you are an experienced pilot, how can you
avoid the stingers ?
Let s say you try to push a large mass like
an Anvil.
Is it harder to push it on Earth or in space,
out of gravity reach ? (tricky)
F = ma
Newton's second law
Push/pull = change in velocity (magnitude or direction or both) but the
Change depends on the mass !!!
Force in newtons, mass in kg, acceleration in m/s/s
Exploration pf physical sciences Newton's law
Sims https://phet.colorado.edu/en/simulation/forces-and-motion-basics
F Δt = m ΔV
FORCE (N) x time (s) = mass (kg) x change in velocity (m/s)
4.1.2. Complete the following statement: The term net force most
accurately describes
a) the inertia of an object.
b) the quantity that causes displacement.
c) the quantity that keeps an object moving.
d) the mass of an object.
e) the quantity that changes the velocity of an object.
15
4.5 Newton’s Third Law of Motion
Newton’s Third Law of Motion
Whenever one body exerts a force on a
second body, the second body exerts an
oppositely directed force of equal magnitude
on the first body.
4.5 Newton’s Third Law of Motion
m
a=
P=
Ma
Newton’s third law of motion
 Newton’s 3rd Law of Motion states that
forces always occur in pairs:
 When one object exerts a force on a second
object, the second exerts an equal and
opposite force on the first.
34
Newton’s third law of motion,
cont’d
 If object A exerts a force on object B, then
object B exerts an equal force in the
opposite direction on A:
 FA on B = - FB on A
When you fall down, you feel the Earth
exerting a force on you but you also
exert that same force on the Earth.
35
4.7.7. Ryan walked to a cliff and dropped a stone. Neglecting
any effects due to the air as it falls vertically, which one of the
following is the reaction force to the Earth’s gravity on the
stone?
a) the normal force of the ground below
b) the normal force of Ryan’s hand on the stone
c) the gravitational force of the stone acting on the Earth
d) No reaction force appears in this situation since the stone is
not exerting any forces on anything else.
Newton’s third law of motion,
cont’d
 Think about pushing off against a wall.
 You push against the wall.
 The wall pushes back.
 If the wall is
weak, it might
fall down.
 If not, you move
away.
37
Newton’s third law of motion,
cont’d
 Consider an airplane’s wing.
 Due to the angle of attack, the air impacts the
bottom of the wing.
 The wing pushes the air out of the way.
 The air pushes back and provides some
lift.
38
NEWTON's THIRD LAW
-mA
=F =
Ma
m
a
=
P
=
How Newton's third law can explain
The lift of the space shuttle ?
What happen to your shoulder if you shoot using
a riffle ? To spare your shoulder, is it better to
use a heavy riffle or a light one?
How Newton's third law can explain the walk ?
If you stand on the ground (not on a skateboard)
and you push the wall, why don't you move
back ? What is the other pair of forces
involved ?
In a cowboy movie, the good guy punches the
bad guy who goes flying away. Of course the
good guy stays steady. What is wrong with
that ?
Also, when a bullet hit a person, does it make
sense to see the guy flying off ?
How to relate newton's third law to the rocket ?
M
a
You are traveling in a bus at highway speed on a nice summer day and an unlucky
bug splatters on the front window.
With Newton's third law in mind , Compare to the force that acts of the bug , how
much force acts of the bus ?
Which undergoes the greater acceleration ? Which therefore suffer the greater
damage ?
4.5.2. A cell phone is sitting on a desk. Which one of the
following is the reaction force to the cell phone’s weight on
the desk?
a) the gravitational force on the cell phone
b) the gravitational force on the table
c) the normal force of the Earth on the table
d) the normal force of the cell phone on the table
e) the normal force of the table on the cell phone
4.2.1. A pipe is bent into the shape shown and oriented so that it is sitting
horizontally on a table top. You are looking at the pipe from above. The
interior of the pipe is smooth. A marble is shot into one end and exits
the other end. Which one of the paths shown in the drawing will the
marble follow when it leaves the pipe?
a) 1
b) 2
c) 3
d) 4
e) 5
31
4.1 The Concepts of Force and Mass
Mass is a measure of the amount
of “stuff” contained in an object.
●
Mass does not depend on the
Planet the object is placed.
Unit is kilogram (kg)
● WEIGHT IS THE PULL DUE TO GRAVITY.
WEIGHT is a FORCE but MASS is STUFF(INERTIA)
WEIGHT is measured in Newtons (or pounds).
1 kg has a weight of 9.8N or
●
2.2 Pounds on Earth
●
WEIGHT is a FORCE = pull due to gravity
It depends on the planet, star, object.. you are standing on.
So if your mass is 80kg , your mass on the Moon
Will be : the same ? Greater ? Larger
What about your weight ?
Mass, cont’d
 The weight of an object depends on which
planet you measure the weight.
 The mass of the object is independent of the
planet.
Far from gravity
From planet,
In space, what is
The weight of
The hammer
9
Show applet elevator from website
About 70kg person : And check weight on other planets.
150 pounds on Earth
27 pounds on Moon
400 pounds on Jupiter
2 tons on the Sun
½ ounce on an asteroid (5 pennies)
(6mph to escape, versus 7 miles per second for Earth = 25 200 mph)
1) If a hammer has a mass of 2.5kg, how much does it weighs on eArth ?
On the Moon/ (gravity is divided by 6 on the Moon)
2) What is the mass of a girl who weighs 340N on earth ?
3) what is the mass of a dog that weighs 75N ?
4.2.1. Which one of the following terms is used to indicate the
natural tendency of an object to remain at rest or in motion at
a constant speed along a straight line?
a) velocity
b) equilibrium
c) acceleration
d) inertia
e) force
The net force is the vector sum of all
of the forces acting on an object.
4.3 Newton’s Second Law of Motion
Mathematically, the net force is

written as
F
∑
where the Greek letter sigma
denotes the vector sum.
The net force on an object is the vector sum of
all forces acting on that object.
The SI unit of force is the Newton (N).
Individual Forces
4N
10 N
Net Force
6N
Individual Forces
Net Force
5N
FiND THE NET 
FORCE.
64
3N
DRAW it
Find direction and
Magnitude.
4N
(add 3N@ right and 4N@ up)
A free-body-diagram is a diagram that
represents the object and the forces that
act on it.
Find the net force
4.3 Newton’s Second Law of Motion
The net force in this case is:
275 N + 395 N – 560 N = +110 N
and is directed along the + x axis of the coordinate system.
4.3.4. Which one of the following tools is useful in representing
the forces acting on an object and simplifies problem
solving?
a) free-body diagram
b) scalar drawing
c) vector analyzer
d) Newton’s ladder
e) force monitor
In a free-body diagram you find the components of the forces.
The x-axis is independent from the y-component
1) you are dragging a crate and the rope makes a 25 degrees angle with horizontal.
The tension in the rope is 250N. Make a free-body diagram and show the force.
A) How much force is being used to drag the crate ? (find the horizontal component)
B) How much force (from you) is pulling the crate upward ?
25
2) Using a window pole that makes an angle of 23 with the window, you push up on the pole with
a force of 85N to close up the window. Make a free-body diagram and show the force.
A) Find the effective force that is pushing the window up
B) the force pushing the window against the sash
3) You push a lawn mower with a force of 160N, exerted directly along its shaft.
The shaft makes an angle of 55 degrees with the ground.
Make a free body-diagram
A) How much force is moving the lawn mower (along horizontal)
B) how much force is pushing the lawn mower toward the ground.
In which direction do you think the boat will move ? =
Draw the resultant Fnet ( a vector). The boat will move in the same
Direction as Fnet. (Newton's second law)
If |F| = 100N
Find R or Fnet
First draw a free-body
Diagram :attach the tails
Of the forces at the origin.
The boat is reduced to
The origin.
ITS EASY TO ADD VECTORS IF THEY ARE PERPENDICULAR
A = 12N @ 20
B= 25N @60
B
Consider these 2 vectors A and B
How can you add them using geometry ?
Using Algebra ?
25
60
20
12
A
Rx= ____, Ry = ____ so magnitude of R = ________
Adding vectors
49
When you add vectors you add the components
See exploration of physics.
How to add vector ?
Use the component method.
A = 12N @ 20
B= 25N @60
A+ B = R
A
Rx=Ax + Bx
Ry = Ay + By
To draw R move B on the head of A
And connect the tail of A to the head of B
Fill the table:
25
X
60
12
20
B
A
B
R
Direction = arctan (Y/X) =
Y
The raft is acted upon by 2 forces:
from the water pushing @ right and from the sailor
Find the net force acting on the boat.
A
A=15N@67
B=17N@0
B
First build a free-body diagram
Attach the tails to the origin.
Find the net force Fnet = R = A +B
X
A
B
R=Fnet
Direction = arctan (Y/X) =
Y
Examples of nonfundamental forces:
friction
tension in a rope
normal or support forces
●
Weight is a fundamental force
●
Weight W is straight down to center of Earth
Mass = 1kg
Weight is 10N@down
W
Normal N
Normal N
Normal force and
Frictional forces
4.8 The Normal Force
Definition of the Normal Force
The normal force is one component of the force that a surface
exerts on an object with which it is in contact – namely, the
component that is perpendicular
to the surface.
4.8.3. What is the meaning of the word “normal” in the term
“normal force?”
a) that it is in magnitude and opposite in direction to the weight
of the object
b) that it is one that is encountered in everyday life
c) that it is directed perpendicular to a surface
d) that it is measurable
e) that it has a magnitude of 1 newton
FIND The Normal Force ?
FN − 11 N − 15 N = 0
FN = 26 N
FN + 11 N − 15 N = 0
FN = 4 N
Example8: a balancing act
In a circus, a woman performs
a headstand on a standing
Performer head. The woman weighs
490 N and the standing performer's
Head and neck weigh 50N.
It is primarily the 7th seventh
Cervical vertebra in the spine that
Supports all the weight above the
Shoulders. What is the normal
Force that this vertebra exerts on the
Neck and the head of the standing
Performer (a)before the act (b)
During the act.
4.8.4. A brick is resting on the surface of a flat board. As one
end of the board is slowly raised, what change, if any, is
there in the normal force exerted on the brick?
a) The normal force increases.
What about the scale?
Same reading ?
Do demo
b) The normal force decreases.
c) The normal force remains constant.
d) Only the direction of the normal force changes.
y
60
x
60
Y
X
Draw the forces applied to the skier.
Then trace the forces in a free-body diagram
Y
Free-body diagram
Normal force Fn
If there is not friction
The magnitude of the acceleration is
10 x sin(angle of the slope)
30
X
Weight mg
30
Draw the forces applied to the skier.
Then trace the forces in a free-body diagram (use the coordinate system X-Y) in red
Show the components of the weight in blue.
Fn
30
mg
Mass = 32kg
Find the x-component of weight
Find the y-component of weight
ur
pulls
P
4.8.1. A free-body diagram is shown for the following situation: a force
on a crate of mass m on a rough surface. The diagram shows the
magnitudesurand directions of the forces that act on the crate
in this
r
situation. F represents the normal
r force on the crate, g represents the
N
acceleration due to gravity, and represents the frictional force. Which
f
one of the following expressions is equal to the magnitude of the normal
force?
a) P − f / µ
b) P − f
c) P − f − mg
d) mg
e) zero
4.10 The Tension Force
Cables and ropes transmit
forces through tension.
4.10 The Tension Force
A massless rope will transmit
tension undiminished from one
end to the other.
If the rope passes around a
massless, frictionless pulley, the
tension will be transmitted to
the other end of the rope
undiminished.
demo
http://www.walter-fendt.de/ph14e/equilibrium.htm
EQUILIBRUM MEANS : no acceleration
MEANS :The net force is 0
Means Sum of x-component = 0
Sum of y-components = 0
F1
F2
   
∑ F = F1+F2+F3 = 0
F1x+F2x+F3x=0
F1y+F2y+F3y=0
F3
http://www.walter-fendt.de/ph14e/equilibrium.htm
68
4.11 Equilibrium Application of Newton’s Laws of Motion
First draw a free-body diagram
Then find T1 and T2
1) |T2| is a tension so |T2| = 2.2x10 = weight of the mass = |T1|
F
T1
T2
x-axis
Fx
y-axis
Fy
4.11 Equilibrium Application of Newton’s Laws of Motion


+ T1 sin 35 − T2 sin 35 = 0


+ T1 cos 35 + T2 cos 35 − F = 0
1) find T1 and T2
You can use wolfram
T3=200N
2) what will happen if the string
Is pulled until horizontal.
http://www.wolframalpha.com/examples/Algebra.html
x-axis
y-axis
0
0
T1
T2
T3
SUM=FNET
1) On a camping trip, you stretch a rope between 2 trees and hang your backpack from
the middle of it. To keep it safe from bears. The mass of the backpack is 32kg. Each
half of the rope makes a 40 degrees angle with the horizontal.
x-axis
Find the tension T in the rope.
T
T
T
y-axis
T
mg
W
SUM=FNET
2) A 35 kg child is on a swing supported by 2 ropes. A baby sitter
is holding the swing so that The ropes make an angle of 25
degrees with the vertical. How much tension is in each rope.
Here is the free body diagram.
T
T
F
W
w
y-axis
0
0
T
T
SUM=FNET
http://www.wolframalpha.com/examples/Algebra.html
T2
3)
Write the 2 equations with 2 unknowns
with T1 and
T2
x-axis
mg
T
F
0
F
25
T
0
T1
x-axis
y-axis
0
0
T1
T2
mg
SUM=FNET
Y
7)
A wagon weighs 40 lb and the angle that the hill makes with the
horizontal is 35 degrees
The weight of the wagon has
Y 2 components. In the (X,Y)
coordinate system Below trace the x-component if the weight of
the wagon and its y-component.
X
35
35
w
X
!!!Same angle !!!
w
A)Find the force pushing the wagon downhill .(that is the x-component of the weight !)
B) The force pushing the wagon into the surface (that will be the y-component of the weight !!)
C) if the wagon is moving at a constant speed Find the normal force N (recoil force from ground)
and the pulling force F. I did the FBD for u
(hint: equilibrium So sum of y-components = 0 and sum of x-components = 0 see diagram below)
!! SUPPOSE THE PULLING FORCE ALONG THE PLANE !!
Y
x-axis
N
X
F
mg
SUM=FNET
N
y-axis
F
w
0
0
8) refer to 7) to draw the free-body diagram
A well-oiled, frictionless wagon of mass 75kg is pulled uphill, using a force of only 110N.
A) draw a free-body diagram
9) A force of 40lb is needed to push a wagon up a 35 degrees . Neglect
friction. How much does the wagon weighs ?
10) Write the 2 equations that will give you the
Unknown TR and TL
Frictional forces
Static friction
Kinetic friction
http://phet.colorado.edu/en/simulation/forces-and-motion-basics
4.9 Static and Kinetic Frictional Forces
When an object is in contact with a surface there is a force
acting on that object. The component of this force that is
parallel to the surface is called the
frictional force.
4.9 Static and Kinetic Frictional Forces
When the two surfaces are
not sliding across one another
the friction is called
static friction.
4.9 Static and Kinetic Frictional Forces
Static friction opposes the impending relative motion between
two objects.
Kinetic friction opposes the relative sliding motion motions that
actually does occur.
f k = µ k FN
0 < µ <1
is called the coefficient of kinetic friction.
4.9 Static and Kinetic Frictional Forces
Note that the magnitude of the frictional force does
not depend on the contact area of the surfaces.
4.9 Static and Kinetic Frictional Forces
The sled comes to a halt because the kinetic frictional force
opposes its motion and causes the sled to slow down.
Suppose the coefficient of kinetic friction is 0.05 and the total
mass is 40kg. What is the kinetic frictional force?
AIR RESISTANCE
And terminal speed
See exploration of physical
81
DRAW a free-body diagram for :
1) a parachutist speeding down to Earth (include air resistance)
2) a cockroach falling from the empire state building at a constant speed
3) A boy leaning with one hand on a tree. He stands on the ground
(hint: all force come in pair)
4) an airplane flying. Included lift force
(from air pushing up) and thrust (motor).
The plane moves at a constant speed
5) a block pulled by a string and sliding. (include friction)
Speeding up
6) a couch pushed over the floor
7) block on an inclined planed not sliding
Y
Draw free-body diagram
But include friction.
He is sliding at a constant speed.
X
Draw the forces applied to the skier.
Then trace the forces in a free-body diagram
FN
fk
Y
FN
fk
X
W
Angle slope
The effect that a force has in any direction can be found by calculating its component in that direction
4) A2.5kg brick is being pulled by a cord (at a constant speed)
that makes an angle of 20 degrees with the horizontal and has a 7N of tension in it.
The forces are: tension, weight, normal, friction
A) Find the force of friction
B) the elastic recoil of the table (normal)
20
5) You are pushing a lawn mower at a constant speed. (so it is an equilibrium situation).
The mass of the mower is 22 kg and is being pushed against a frictional resistance of 150N.
The shaft makes an angle of 30 degrees
with the horizontal. So the forces are the push down along the shaft, the normal from the ground (recoil),
the weight upward
And the friction opposing the motion along the ground.
A) Find the compression in the shaft (that is the push ). Focus on the horizontal.
B) Find the elastic recoil of the ground (that is the normal force)
6) A helium balloon is in equilibrium as shown. The balloon weighs 25N and is acted
Upon by 4 forces. The weigh (down), the buoyancy (up) the tension in
The rope that is holding down the balloon. The force of the wind pushing the
Balloon to the right. The rope makes an angel of 20 degrees with the
Vertical. The tension is 16N.
Find the buoyancy and the force exerted by the Wind.
17) A sled is being pulled along a horizontal road at a constant speed by means
of a rope that makes an angle of 25 degrees with the horizontal.
If the friction between the sled and the snow is 85N, how much is the tension in the rope ?
(free-body diagram please)
18) This block is not moving on the inclined plane. If the weigh is
10N and the angle of the plane With horizontal 20 degrees:
A) find the frictional force f
B) find the normal N (recoil force from the ground)